1. Field of the Invention
The present invention relates to a read circuit, a variable resistive element device, and an imaging device. In particular, the present invention relates to a reset operation of an integration circuit in a read circuit that uses the integration circuit for sensing a current flowing through a variable resistive element.
2. Description of Related Art
A technique for sensing a current flowing through a variable resistive element whose resistance is variable is one of techniques applied to various technical fields. One of applications of this technique is an imaging device using a thermoelectric conversion element such as a bolometer. The thermoelectric conversion element is an element in which a temperature variation associated with an incidence of infrared light appears as a variation in its resistance, and is sensitive to the incident infrared light. The imaging device is provided with the thermoelectric conversion elements that are one-dimensionally or two-dimensionally arranged. An infrared image can be obtained by applying a predetermined bias voltage to each thermoelectric conversion element of the imaging device and thereby sensing a current flowing through the each thermoelectric conversion element.
One of methods for sensing a current flowing through a variable resistive element is to integrate the current flowing through the variable resistive element with the use of an integration circuit to extract it as a voltage signal. Japanese patent No. 3821040 discloses an imaging device provided with a read circuit that senses a current flowing through a thermoelectric conversion element based on such a method.
FIG. 1 is a conceptual diagram illustrating a configuration of the imaging device, particularly a configuration of a readout circuit 134, disclosed in Japanese patent No. 3821040. The imaging device has: a plurality of read circuits 134; switches 135; an output buffer 136; and three source followers 137 to 139 provided in series along a feedback loop of the output buffer 136. The switches 135 are used for selecting any of the plurality of read circuits 134. The output buffer 136 performs a voltage follower operation and outputs an output signal VOUT whose voltage level is the same as that of a voltage signal received from the selected read circuit 134.
The read circuit 134 performs an operation for sensing a current flowing through a thermoelectric conversion element 102. In FIG. 1, only one thermoelectric conversion element 102 is illustrated; however, it should be appreciated that a number of thermoelectric conversion elements 102 are actually arranged on a substrate, and the read circuit 134 senses a current flowing through the thermoelectric conversion element 102 selected by a pixel switch 101 and a horizontal switch 103.
The read circuit 134 includes a bias circuit 141, a bias cancel circuit 142, and an integration sample hold circuit 131. An output of the integration sample hold circuit 131 is connected to the switch 135 through two source followers 132 and 133.
The bias circuit 141 has a function of applying to the thermoelectric conversion element 102 the same bias voltage as a bias voltage VBOL applied to a bias voltage terminal 108. More specifically, the bias circuit 141 includes an integration transistor 104, an operational amplifier 105, a switch 106, and a bypass capacitor 107. As the integration transistor 104, an N-channel MOSFET is used. A gate of the integration transistor 104 is connected to an output of the operational amplifier 105, and the switch 106 and the bypass capacitor 107 are connected in parallel between the gate and a source of the integration transistor 104. An inverting input of the operational amplifier 105 is connected to the source of the integration transistor 104, and a non-inverting input terminal of the operational amplifier 105 is connected to the bias voltage terminal 108.
The operational amplifier 105 plays a role of compensating a gate-source voltage drop in the integration transistor 104. As is widely known, when an N-channel MOSFET is in the ON state, a voltage drop comparable to its threshold voltage appears between the gate and the source. The operational amplifier 105 compensates the voltage drop in the integration transistor 104, which enables application of the same bias voltage as the bias voltage VBOL to the thermoelectric conversion element 102.
The switch 106 is provided for preventing the operational amplifier 105 from oscillating when all the thermoelectric conversion elements 102 connected to the read circuits 134 are in non-selected state (for example, a period when all the horizontal switches 103 are turned OFF or when all the pixel switches 101 are turned OFF). The switch 106 is provided between the gate and the source of the integration transistor 104, and is turned ON during a period when the thermoelectric conversion elements 102 are in the non-selected state. In this case, the output of the operational amplifier 105 is connected to the inverting input thereof, and thus the operational amplifier 105 operates as a voltage follower. This prevents the operational amplifier 105 from oscillating.
The bypass capacitor 107 prevents the operational amplifier 105 from oscillating that may occur if a defect in a manufacturing process causes an open fault of the thermoelectric conversion element 102 and a bias current does not flow. If the bias current does not flow in the thermoelectric conversion element 102, that is, if a drain current does not flow in the integration transistor 104, a transconductance gm of the integration transistor 104 is reduced and a resistance component of a feedback loop of the operational amplifier 105 is increased. In this case, a phase delay at the feedback loop is increased and thus a phase margin of the operational amplifier 105 may not be ensured. The bypass capacitor 107 provided between the gate and the source of the integration transistor 104 plays a role of ensuring the phase margin.
The bias cancel circuit 142 is used for cancelling the drain current of the integration transistor 104, i.e., an offset component of a current flowing through the thermoelectric conversion element 102. To cancel the offset component is effective for effectively using a dynamic range of the integration sample hold circuit 131.
More specifically, the bias cancel circuit 142 includes a canceller resistor 109, a canceller horizontal switch 110, a canceller transistor 111, an operational amplifier 112, and a switch 113. As the canceller transistor 111, a P-channel MOSFET is used. The canceller resistor 109 and the canceller horizontal switch 110 are connected in series between a power supply line VCC to which a power supply voltage is supplied and a source of the canceller transistor 111. The switch 113 is connected between a gate and the source of the canceller transistor 111, and a drain of the canceller transistor 111 is connected to the drain of the integration transistor 104. The gate of the canceller transistor 111 is connected to an output of the operational amplifier 112. An inverting input of the operational amplifier 112 is connected to the source of the canceller transistor 111, and a non-inverting input terminal of the operational amplifier 112 is connected to a voltage terminal 114 to which a bias cancel voltage VCAN is supplied. A current flowing in the bias cancel circuit 142 is controlled in response to the bias cancel voltage VCAN. As in the case of the operational amplifier 105 of the bias circuit 141, the operational amplifier 112 is provided for the purpose of compensating a gate-source voltage drop in the canceller transistor 111.
As in the case of the switch 106 of the bias circuit 141, the switch 113 of the bias cancel circuit 142 is provided for preventing the operational amplifier 112 from oscillating when all the thermoelectric conversion elements 102 connected to the read circuits 134 are in non-selected state. When all of the thermoelectric conversion elements 102 connected to the read circuits 134 are in the non-selected state, a current in the bias circuit 141 is shut off, which will result in no path through which a current in the bias cancel circuit 142 can flow. For this reason, the canceller horizontal switch 110 is turned OFF in synchronization with the horizontal switch 103, and the current in the bias cancel circuit 142 is shut off. The switch 113 provided between the gate and the source of the canceller transistor 111 is turned ON during the period when all the thermoelectric conversion elements 102 are in the non-selected state. In this case, the output of the operational amplifier 112 is connected to the inverting input thereof, and thus the operational amplifier 112 operates as a voltage follower. This prevents the operational amplifier 112 from oscillating.
The integration sample hold circuit 131 includes an integration operational amplifier 116, two integration-hold capacitors 119 and 122, switches 118, 120, 121 and 123, a reset switch 125, source followers 128 and 129, and a selection switch 130. The integration sample hold circuit 131 is configured to use one of the integration-hold capacitors 119 and 122 as an integration capacitor and to use the other one as a hold capacitor that holds a voltage signal obtained by an integral operation. The selection switch 130 is used for selecting the integration-hold capacitors 119 and 122. If the integration-hold capacitor 119 is selected by the selection switch 130, a voltage signal OUTA held by the integration-hold capacitor 119 is output to the source follower 132 through the source follower 128. On the other hand, if the integration-hold capacitor 122 is selected, a voltage signal OUTB held by the integration-hold capacitor 122 is output to the source follower 132 through the source follower 129. A reset operation of the integration sample hold circuit 131 is performed by turning ON the reset switch 125 with the integration-hold capacitor used as the integration capacitor being electrically connected between an input and an output of the integration operational amplifier 116.
The reset operation of the integration sample hold circuit 131 is performed during the period when all of the thermoelectric conversion elements 102 are in the non-selected state. It should be noted here that the bias circuit 141 and the bias cancel circuit 142 both are in invalid states during the period when all the thermoelectric conversion elements 102 are in the non-selected state. For this reason, a switch 115 for separating the integration sample hold circuit 131 from the bias circuit 141 and bias cancel circuit 142 is provided in the read circuit 134 of FIG. 1 in order to surely perform the reset operation. The switch 115 is turned OFF during when the reset operation of the integration sample hold circuit 131 is performed.
Japanese Laid-Open Patent Application No. 2007-033177 discloses a similar configuration of a read circuit. Specifically, the document discloses a read circuit in which an integration circuit and a sample hold circuit are used instead of the integration sample hold circuit 131.
The inventor of the present application has recognized the following points. One problem of the read circuit 134 shown in FIG. 1 is that a gate voltage of the integration transistor 104 can excessively increase. If a high-temperature subject is imaged, an output of the integration operational amplifier 116 is saturated at an upper limit of an output dynamic range, and an imaginary short of the integration operational amplifier 116 is broken. If the imaginary short of the integration operational amplifier 116 is broken, a drain voltage of the integration transistor 104 is decreased and hence a drain-source voltage is decreased. Meanwhile, the integration transistor 104 and the operational amplifier 105 attempt to supply a current corresponding to the bias voltage VBOL to the thermoelectric conversion element 102, and therefore the gate voltage of the integration transistor 104 is increased. In the worst case, the gate voltage is raised to an upper limit of an output dynamic range of the operational amplifier 105. The gate voltage of the integration transistor 104 recovers to a normal operating voltage by the reset operation of the integration sample hold circuit 131; however, it takes some time to recover to the normal operating voltage. This can cause increase in a settling time of the reset operation of the integration sample hold circuit 131.
A similar problem can also arise in the bias cancel circuit 142. If a low-temperature subject is imaged, an output of the integration operational amplifier 116 is saturated at a lower limit of the output dynamic range, and the imaginary short of the integration operational amplifier 116 is broken. If the imaginary short of the integration operational amplifier 116 is broken, a drain voltage of the canceller transistor 111 is increased and hence a drain-source voltage is decreased. Meanwhile, the canceller transistor 111 and the operational amplifier 112 attempt to supply a current corresponding to the bias cancel voltage VCAN to the thermoelectric conversion element 102, and therefore a gate voltage of the canceller transistor 111 is decreased. In the worst case, the gate voltage is decreased to a lower limit of an output dynamic range of the operational amplifier 112. The gate voltage of the canceller transistor 111 recovers to a normal operating voltage by the reset operation of the integration sample hold circuit 131; however, it takes some time to recover to the normal operating voltage. This can cause increase in the settling time of the reset operation of the integration sample hold circuit 131.